1. Field of the Invention
The present invention relates to an image memory device for taking the synchronization of image signals with different timings which signals appear in, for example, a frame synchronizer, a vision mixer, a video switcher or the like.
2. Description of the Prior Art
Firstly, a motion picture discontinuity phenomenon which may occur in the case where a motion picture video signal is asynchronously input and output, will be explained by virtue of FIGS. 3 to 5. Generally, in the case where the read and write operations are asynchronously conducted, a frame passing phenomenon may take place, since there is almost no complete coincidence between the read and write frequencies. Now consider the case where the writing side will soon come up with the reading side taken as a reference. In this case, since the writing for a certain line in a memory is made after the reading of that line, as shown in FIG. 3, the reading on the reading side always precedes by one field as compared with the writing on the writing side. Next, consider the case where the writing side passed the reading side ahead, as shown in FIG. 4. In this case, since the reading is made immediately after the writing, the write operation and the read operation make access to the same field. Therefore, old data and new data co-exist in the output of a field in which the passing has occurred. Accordingly, in the case of a motion picture, there results in a discontinuous image. Especially, in the case of an output signal of a VTR, since it contains jitters, the passing continuously takes place, thereby resulting in a disturbed image.
In the conventional technique, a line memory 5.3 is provided separately from a field memory 5.2, as shown in FIG. 5, in order to avoid the above-mentioned discontinuity phenomenon. Namely, the phase of a vertical synchronizing signal on the input side and that of a vertical synchronizing signal on the output side are compared with each other at the beginning of each field. In the case where a difference between both the phases becomes smaller than 1H, a selector 5.4 is switched so that data of a line preceding a current line by 1H are read and delivered to a serial output memory, thereby avoiding the passing. Next, in order to take a time matching, a signal delayed by 1H is read from the line memory at a synchronized timing so that no discontinuity is produced.
In the conventional method, however, the line memory 5.3 is required in addition to the field memory 5.2. Therefore, this method is not satisfactory in the aspect of cost and space.